1. Field of the Invention
This invention relates to a method and apparatus for measuring the weight of a seat occupant and classifying the occupant into a weight class.
2. Related Art
Most vehicles include airbags and seatbelt restraint systems that work together to protect the driver and passengers from experiencing serious injuries due to high-speed collisions. It is important to control the deployment force of the airbags based on the size of the driver or the passenger. When an adult is seated on the vehicle seat, the airbag should be deployed in a normal manner. If there is an infant seat or small adult/child secured to the vehicle seat then the airbag should not be deployed or should be deployed at a significantly lower deployment force. One way to control the airbag deployment is to monitor the weight of the seat occupant.
Current systems for measuring the weight of a seat occupant are complex and expensive. These systems use various types of sensors and mounting configurations to determine seat occupant weight. For example, some systems use pressure sensitive foil mats or a plurality of individual sensors mounted within a seat bottom foam cushion while other systems mount sensors on seat tracks, seat frame members, or other seat structural members. The combined output from the sensors is used to determine the weight of the seat occupant. The accuracy of the weight measurements from these types of sensor systems can be compromised due to additional seat forces resulting from the occupant being secured to the seat with a seatbelt.
For example, weight sensor systems can have difficulty identifying an adult, a child, or a car seat when the seatbelt is being used. When a child seat is secured to a seat with a seatbelt, an excess force acts on the sensors mounted within the rear portion of the seat bottom, which interferes with accurate weight sensing. Over tightening of the seatbelt to securely hold the child seat in place, pulls the child seat down against the rear part of the seat bottom, causing the excessive force measured by the sensors. Due to this effect, the current weight sensing systems have difficulty in discerning between an adult belted to a seat and a child seat secured to the seat with a seatbelt.
In order to address this problem, sensors have been incorporated into the seatbelt to measure the tension force applied to the seatbelt as passengers or a child seat is secured to the seat. High seatbelt tension forces indicate that a child seat is secured to the seat. One disadvantage with current seat belt force sensors is that it is difficult to get accurate seat belt force measurements. Another disadvantage with current seat belt force sensors is that non-axial loading on the belt can affect the accuracy of the force measurement.
Once seat occupant weight force measurements and belt force measurements are taken, the seat occupant is typically classified into a predetermined classification. Some systems attempt to classify seat occupants into predetermined customer-specified classes usually based only on occupant weight. The classification information is then used to modify the deployment of the airbag. These systems do not provide accurate and consistent classification over a wide range of adverse road conditions and/or occupant seating conditions.
The accuracy of the weight measurements from known sensor systems can also be compromised due to variable seat forces resulting from momentary events such as rough road conditions or seat occupants adjusting seat position, for example. These types of events can transfer or remove weight from the seat for short periods of time, which affects the accuracy of the system.
Thus, it is desirable to have an improved seat occupant weight measurement and classification system that provides increased accuracy in weight measurement and classification as well as overcoming any other of the above referenced deficiencies with prior art systems.
A weight classification system utilizes weight and seat belt force sensors to classify seat occupants into a predetermined weight classification. Preferably, a plurality of weight force sensors are mounted between a seat bottom and a vehicle structure and a seat belt force sensor is installed within a seat belt assembly. The weight sensors are preferably located at four connecting points for the seat frame and pick up all distributed forces (positive and/or negative forces) to determine the total weight on the seat and the center of gravity. The center of gravity and total weight determinations are used to classify the seat occupant, which is further used to control deployment of a safety device, such as an airbag.
In the preferred embodiment, one weight sensor is mounted at each corner of the seat bottom. Each weight sensor includes a bending element having one end mounted to the seat frame and an opposite end mounted to a vehicle or other seat structure such as a riser, seat track, or vehicle floor, for example. At least one strain gage assembly is mounted on a center portion of the bending element and an integrated electronics package electrically connects the strain gage to an electronic control unit (ECU) or other similar device. The strain gages measure deflection of the center portion and generate a weight signal that is sent to the ECU.
Preferably, the bending element includes a top surface and a bottom surface with at least one centrally formed groove in one of the top or bottom surfaces. The groove extends at least partially along the width of the sensor to localize strain in the center portion. The strain gage is placed on the other of the top or bottom surfaces, in an opposing direction from the groove.
Output from the sensors near the front of the seat bottom are combined and compared to output from the sensors near the rear of the seat bottom to determine the center of gravity. The initial seat occupant weight can be adjusted to take into account the center of gravity of the seat occupant. The initial seat occupant weight can also be adjusted to take into account forces exerted on the seat occupant by the seat belt assembly.
The seat belt sensor includes a load cell with a strain gage that is integrated into a seat belt mechanism that is used to secure an occupant to a vehicle seat. When the seat belt is tightened, the sensor is pulled into tension and this is measured by the strain gage. The strain gage measurements and signals are sent to the ECU. The ECU uses the information to determine whether a child seat or an adult is secured to the vehicle seat. Further, the seat belt force information is used to adjust the initial seat occupant weight and is used to properly classify the seat occupant.
The seat occupant is classified into one of several different weight classes based on an estimated value of the seat occupant weight. Each of the weight classes has upper and lower thresholds that define the class. Over time, several comparisons are made between the estimated weight and the thresholds of the weight classes and each comparison results in a weight class sample. The seat occupant is assigned a specific weight class designation once a predetermined number of consistent and consecutive weight class samples is achieved. The specific weight class designation remains locked until a certain number of inconsistent weight class samples are observed.
In a disclosed embodiment of this invention, the method for classifying a seat occupant into a weight class includes the following steps. The seat occupant weight is measured resulting in an estimated weight. The estimated weight is compared to a series of weight classes with thresholds to determine a class sample. The previous steps are repeated until a predetermined number of class samples having the same value is achieved and the class sample becomes locked as the occupant weight class.
Additional steps include generating an occupant weight class signal corresponding to the locked occupant weight class, transmitting the occupant weight class signal to a control unit, and modifying deployment of an airbag based on the occupant weight class signal. The weight class is unlocked when a predetermined number inconsistent class samples is observed. When the class is unlocked, the process repeats.
Once the occupant has been classified into a weight class, that class becomes the known class for the next comparison. Preferably, each weight class is assigned an upper threshold and a lower threshold. At each iteration, the estimated weight is compared to the upper and lower thresholds for the last known weight class. The new class sample is designated the same as the last known weight class if the estimated weight is between the upper and lower thresholds for the last known weight class. The sample is set equal to a next higher weight class if the estimated weight is greater than the upper threshold for the last known weight class or the class sample is set equal to a next lower weight class if the estimated weight is less than the lower threshold for the last known weight class.
In one disclosed embodiment, the value of the upper threshold of the class sample is increased by a first predetermined amount and the value of the lower threshold of the class sample is decreased by a second predetermined amount after the class sample is locked. The upper and lower thresholds are returned to their initial values when the class sample becomes unlocked.
The subject invention uses seat occupant weight and seat belt force measurements in combination with varying weight class thresholds and class sample histories to produce a more stable, accurate and robust classification process that reduces errors caused by changes in occupant seating position and adverse road conditions. The more accurate classification system is used to generate control signals, which are used to modify airbag deployment.